Pressure-Induced Spin-State Crossovers in Six-Coordinate FeIILnL‘m(NCS)₂Complexes with L = L‘ and L ≠ L‘:  A XANES Investigation

The pressure-induced iron(II) high-spin (HS) → low-spin (LS) conversion has been investigated, by near-edge X-ray absorption (XANES) spectroscopy at room temperature, in a number of parent six-coordinate complexes of the type FeL n L‘ m (NCS) 2, viz. Fe(phen) 2 (NCS) 2 form I ( 1 ) and form II ( 2 ), Fe(py) 2 bpym(NCS) 2 ( 3 ), Fe(py) 2 phen(NCS) 2 ( 4 ) and Fe(py) 4 (NCS) 2 ( 5 ), where phen = 1,10-phenanthroline, py = pyridine, and bpym = 2,2‘-bipyrimidine. The spectra of the two spin isomers are interpreted. For compounds 1 − 4, known to exhibit thermally-induced spin transitions, n LS vs P plots ( n LS = LS fraction, P = pressure) are centered around P c = 0.80 ( 1 ), 0.65 ( 2 ), 1.00 ( 3 ) and 1.55 ( 4 ) GPa. For 5, which retains the HS form at any temperature under atmospheric pressure, the P c value is much higher than the previous ones, lying in the range 5−7 GPa. After pressure was released, the spectra were found to be quite similar to those obtained before pressure was applied. The P c values of 1 − 4 were analyzed in terms of spin-transition temperatures ( T c ), entropy (Δ S ) and crystal volume (Δ V SC ) variations associated with the spin change, crystal structures, and volumic compressibility coefficients ( k v ), insofar as these data were available. The alteration of the spin-conversion development, when subjecting a compound (here 2 ) several times to increasing then to decreasing pressures, was ascribed to a progressive decrease in the number of crystal defects (vacancies in particular).